(1) Field of the Invention
This invention relates to yarns for use in papermaking fabrics, and more specifically to stuffer yarns used in papermakers"" fabrics.
(2) Description of Prior Art
In the early days of papermaking, a paper-forming slurry of particles was deposited on a wire screen. Eventually, that wire screen evolved into a woven fabric woven from yarns. Indeed, because the fabrics are woven, such products that are used on papermaking machines have become known as papermachine clothing. As one can imagine, the properties of the yarns used in weaving papermachine clothing are important, and contribute to the final characteristics of the paper itself.
The usual papermaking machine has three primary sections: a forming section, a press section, and a drying section. In the forming section, a water slurry or suspension of cellulose fibers, known as the paper stock or pulp, is fed onto the top of the upper run of a traveling endless forming belt. The forming belt provides a papermaking surface and operates as a filter to separate the cellulose fibers from the aqueous medium to form a wet paper web. In forming the paper web, the forming belt serves as a filter element to separate the aqueous medium from the cellulose fibers by providing for the drainage of the aqueous medium through its mesh openings, also known as drainage holes, by vacuum means or the like located on the drainage side of the fabric.
From the forming section, the somewhat self-supporting paper web is transferred to the press section of the machine and onto a press felt, where still more of its water content is removed by passing it through a series of pressure nips formed by cooperating press rolls, these press rolls serving to compact the web as well. A press felt generally includes a woven fabric to which a batt material is applied, usually by one or more needling operations, as is known in the art. As will be described herein, the stuffer yarns of the present invention may be used to enhance batt anchorage in a press felt.
After leaving the press section, the paper web is transferred to a dryer section where it is passed about and held in heat transfer relation with a series of heated, generally cylindrical dryer rolls to remove still further amounts of water therefrom. One or more dryer fabrics may be employed to press the moist web uniformly and successively against the dryer cylinders to dry the web. As used herein and in the claims, the term xe2x80x9cpapermaking machinexe2x80x9d is to be considered in a broad or generic sense, that is, the machine producing a paper or paper-like material such as pulp, board, wet laid non-woven sheet or other similar structures.""
In the dryer section, the dryer cylinders are internally heated by steam or the like. The cylinders usually have imperforate surfaces for contacting the paper web. Other rolls, such as pocket rolls, may have surfaces that are perforated or slotted to permit the passage of heated air there through to increase the drying action on the web.
Ideally, dryer fabrics should have at least the following properties. First, they should have a top surface that is fine enough to minimize marking of the sheet of paper being produced. Second, they should have a resilient bottom layer to provide long life while enduring the stress the fabric is subjected to while in contact with the machine over a long period of time. Third, the dryer fabric weave should be open enough to allow heat to pass through without significant impedance. Fourth, the fabric should be designed in such a way that the permeability of the fabric, and thus the heat transfer from the dryer cylinders to the web, may be controlled.
In multi-layer dryer fabrics, it is known in the art that a certain degree of control may be exhibited over the permeability of the woven dryer fabric by inserting additional cross machine direction yarns, called stuffer picks, or stuffer yarns, into the weave at selected positions across the fabric. These yarns serve to fill the air pockets or voids created in the weave between the machine direction and cross machine direction yarns. These stuffer yarns can also serve the supplemental purpose of joining the top and bottom layers of the fabric and lending an increased cohesiveness and durability to a fabric that would otherwise be overly porous and vulnerable to wear.
In the past, several varieties of stuffer yarns have been employed for the purposes noted above. The most common yarns utilized have been cabled monofilament yarns, hollow monofilament, and thermoplastic coated (or deformable) cross-machine-direction yarns. Each of these yarns brings limitations to the dryer fabric application. Specifically, a daunting problem is that none of the three prior art stuffer yarns provide the desired degree of permeability control in the dryer fabric in which a stuffer yarn is used.
In addition, a number of problems specific to the use of cabled yarns as stuffer yarns in a dryer fabric are well known in the art. For example, a major problem is that they are bound tightly and are not able to efficiently fill the interstices of the woven fabric to impede the flow of air, as desired. Furthermore, the cabled monofilament stuffer yarn does not weave efficiently. Undesirable torque builds up during the weaving process that results in pigtails or kinks being pulled into the fabric.
In addition, the diameter and shape of the fibers used in the manufacture of the cabled yarn should be identical. When fibers of varying diameters are twisted into a cabled yarn, the resulting yarn becomes buckled and kinked. The difficulties of incorporating such a yarn into a weave are obvious. Even if it were possible to weave such a yarn into a fabric, the resulting fabric would be uneven and cause marking and non-uniform drying to the paper web. Thus, cabled yarns are limited to fiber bundles of uniform cross section and diameter. The result of this limitation is that cabled yarns have a somewhat uniform radius, and will not fill fabric voids. The result of having fabric voids in papermakers"" fabric made with cabled yarns of the prior art is less control over the permeability of the fabric.
Monofilament hollow yarns have also used been used as cross machine direction stuffer picks in dryer fabrics. Further, while hollow monofilaments will distort to fill the fabric voids they will not provide the same effect as the inter-twisted yarns"" ability to allow individual monofilaments to disperse in the fabric voids. The major disadvantage of using hollow yarns as stuffer yarns is that such yarns are more difficult to produce than conventional monofilament yarns and, as a result, are significantly more expensive than cabled monofilament or inter-twisted monofilament yarns.
Cross-machine direction yarns that are deformable have been used as stuffer yarns, but like hollow yarns, they are very costly to manufacture. Further, they require special post-treatment to allow the coating to deform to fill the fabric voids.
U.S. Pat. No. 3,675,409 to Rosenstein addressed the problem of pigtails and kinks in a multi-filament fiber bundle, specifically a tow. Rosenstein prevents snags and kinks in a tow through the use of a wrapping filament to keep the tow fibers substantially parallel. The wrapping filaments are wound in a clockwise and counter-clockwise manner to prevent unraveling and loosening of the substantially parallel tow fibers which were then cut for flock. In Rosenstein, the wrapping filaments were limited in their application to ensure that when the tow was chopped they were substantially the same length as the substantially parallel filaments that they bound.
For the foregoing reasons, there is a need for an improved stuffer yarn for use in papermakers"" fabrics, and for an improved method of making such a yarn that is fast and economical.
The present invention satisfies these needs with an improved assembled multifilament stuffer yarn for use in papermakers"" dryer fabrics and press felts, utilizing a plurality of monofilament filaments that are helically wrapped or twisted clockwise and counterclockwise to form a papermakers"" yarn by wrapping or twisting filaments together. A first xe2x80x9ccentral twisting carrierxe2x80x9d serves as the first member of the yarn, and acts as a carrier for the subsequent wrapping of a number of twisting filaments wherein the finished yarn is substantially free of filaments which are parallel.
The yarn is made by passing a first twisting filament or a yarn or a plurality of twisting filaments or a mixture of twisting filaments, hereinafter referred to in the specification and in the claims as the xe2x80x9ccentral twisting carrierxe2x80x9d in sequence through the hollow center of a number of static twisting filament-loaded supply spools positioned sequentially in a linear pattern. The majority of the cross-sectional diameter of the finished yarn is made up of the twisting filaments with substantially no filaments parallel to one another.
As the central twisting carrier is passed through the hollow center of each of the subsequent twisting filament-loaded supply spools, the central twisting carrier is helically wrapped by a twisting filament that is pulled off of the supply spool along with the moving central twisting carrier. The supply spools are oriented in positions such that the twisting filament that is pulled off some of the supply spools wraps the central twisting carrier in a clockwise direction, and some of the twisting filaments wrap the central twisting carrier in a counterclockwise direction.
Accordingly, an object of the invention is to provide an improved assembled monofilament yarn for use in a papermaking fabric that will efficiently control the flow of air through the fabric.
A further object of the present invention is to provide an improved assembled monofilament yarn for use in a press felt that provides improved anchorage for needling batt fibers.
Yet another object of the present invention is to provide an improved dryer fabric for the dryer section of a papermaking or similar machine utilizing the inter-twisted monofilament yarns of the present invention as stuffer yarn.
Another object of the present invention is to provide an improved press felt for the press section of a papermaking or similar machine utilizing the multifilament yarns of the present invention for scrim structure substrate.
Yet another object of the invention is to provide a dryer fabric that may be employed to press the moist web uniformly and successively against the dryer cylinders to dry the web.
A further object of the invention is to provide yarns that can be utilized as stuffer picks in a dryer fabric, filling in fabric voids, thereby allowing for improved control of permeability.
Another object of the invention is to provide a yarn that can be utilized in a papermakers"" fabric and which provides improved control of heat transfer.
Another object of the invention is to provide a yarn that will impart greater wear resistance and compaction resistance to a press felt made therefrom.
Another object of the invention is to provide an assembled monofilament yarn that can be woven efficiently, without undesirable torque build-up that results in kinks being pulled into the fabric.
A further object of the invention is to provide an improved papermakers"" yarn that is less expensive to make than monofilament hollow yarns and other cross-machine-direction yarns.